When Has a Preserve Finished Cooking?

by Rachel Saunders

A preserve may either set or thicken, depending upon its ingredients and the type of preserve it is.

A preserve "sets" when it reaches a high enough temperature to form a jelly when left to cool undisturbed. This temperature (220 degrees F) can only be reached in mixtures containing a high proportion of sugar to moisture. Be careful to avoid cooking any preserve to a temperature higher than 220 degrees F, as this will result in an irrevocably tough, leathery preserve.

Jellies and marmalades, because of their high pectin, sugar, and water contents, must reach the setting point, or they will end up a syrupy mess. However, many jams do not necessarily need to set; because their fruit tends to be less concentrated than a jelly's or a marmalade's, they require much less sugar to have a good flavor. Depending upon the type of fruit, quantity of sugar, and techniques employed, a jam may be more or less jelly-like.

Testing for Doneness
When testing for doneness, remember that most preserves thicken significantly as they cool to room temperature.

There are several ways to see if a preserve has finished cooking. For preserves that are cooked to reach the setting point, you may use a candy thermometer. However, I believe it is important to know what to actually look for to tell if a preserve is done, especially because not all preserves will reach the 220 degrees F setting point. Thus, I prefer a combination of the freezer test and a visual examination of the preserve to test for doneness.

For the freezer test, place a few metal spoons (I suggest five in the recipes) on a saucer in the freezer before you start cooking the preserve. When you think the preserve might be ready, remove it from the heat, take a small representative half-spoonful (one containing both the liquidy and the more solid portions), and carefully transfer it onto one of the frozen spoons. If you are testing a jelly, marmalade, or high-sugar jam, it should resemble a shiny bead of liquid that is resisting the metal of the cold spoon slightly. If it is a low-sugar jam, it may not look as shiny, but it should look cohesive and not watery. Put the cold spoon back in the freezer for three to four minutes. Then, remove it from the freezer and carefully feel the underside of the spoon. It should be neither warm nor cold; if still warm, return it to the freezer for a moment. Tilt the spoon vertically to see if the preserve runs; depending on the individual preserve, it should run either slowly or not at all. If it has not yet reached the appropriate point, bring it back up to temperature, cook it for another three to four minutes, and test again.

Testing for doneness interrupts the heating process, so preserves should only be tested when you think they really are close to being done. Preserves change a lot as they cook. Here is what to look for:

Bubbles and Foam: The fruit often foams a lot during the first stage of cooking; this foam eventually subsides by the time the preserve is done cooking. A preserve's bubbles become progressively less watery and more sugary as it cooks. Depending on the concentration of sugar, the bubbles may become progressively larger and more sputtering (low-sugar jams) or tiny and shiny (most jellies, marmalades, and high-sugar jams).

Appearance: A finished preserve has a slight shine to it, because its concentration of sugar has increased so much during the cooking process. Additionally, the fruit will usually become suspended in the mixture rather than floating to the top; this is often a sign that the proper balance of moisture, acid, and pectin has been reached.

Ability to Sheet: You may test for doneness by dipping a metal spoon in the preserve and holding it perpendicular to the pot at a slight vertical angle while the preserve drips back into the pan. If the preserve is done, the drips will tend to run along the bottom edge of the spoon to collect into one big drip (this is known as "sheeting"). The final drips may tend to cling to the spoon and form little pearl-like drops. This test should always be done in combination with the freezer test, but it is nevertheless a useful way to gauge doneness without interrupting the cooking.

Ability to Set: While doing the freezer test, let the preserve in the pot sit still for a few minutes off the heat to see whether it starts to set. If, after a couple of minutes, the preserve has thickened slightly and started forming a skin across the top, it is likely done. If, however, little liquidy areas have formed on the surface and there is no skin forming, it is probably not ready quite yet.